The present invention relates generally to the measurement of the level of cryogenic liquids in dewars and the like, and more particularly to cryogenic liquid level detector systems utilizing current-carrying superconductor filaments as the detector element.
Liquefied gases such as helium, oxygen, hydrogen and nitrogen are frequently utilized as coolants, fuels, etc., for many processes and apparatus. For example, extensive use is now made of superconducting magnetic coils which must be operated at temperature approaching 0.degree.K. Even superconducting transmission lines are under investigation. A typical coolant for superconducting applications is liquid helium which produces a temperature of about 4.2.degree.K. Because of the heat input to any such system, the helium evaporates and must be replaced. This is normally accomplished by attaching a container (dewar) of liquid helium to the system. Since the cryogenic liquid levels are not easily viewed, some form of level detector must be utilized in all of the systems and dewars. Such a detector provides a signal proportional to the liquid level, and the associated circuitry may initiate a refilling operation when the level reaches a set low point. Generally a detector which provides a continuous indication of the level is preferred over fixed-point detectors. Such a detector is provided by a filament of superconducting material vertically suspended in the dewar and positioned so that the gas-liquid interface in the dewar moves along the filament. A current is passed through the filament and the voltage generated across the filament is a measure of the level of the interface. This voltage is created because the portion of the filament above the liquid level is in the normal resistance state of the superconducting material and exhibits at least a moderate electrical resistance. This is in contrast to the superconducting state where the resistance is substantially zero. Normally a small heater in contact with the filament assures the existence of the normal state above the level. The generated voltage is therefore relatable to the level of the liquid. Further description of a typical cryogenic liquid level detector may be found in my publication "A Superconducting (Nb-Ti) Liquid Helium Level Detector," Advances in Cryogenics Engineering, Vol. 15, p. 124 (1970), Plenum Press. Another description may be found in U.S. Pat. No. 3,267,730.
One concern of any user of a cryogenic system is the minimization of evaporative losses. Although the superconductor filament level detector utilizes only a small current, e.g., &lt;100 mA, the power input due to this current increases the liquid loss. One partial solution known in the art involves periodically, at a fixed time interval, applying the current so that the level is sampled intermittently. Since there is a time delay in reaching a stabilized voltage for a given liquid level each time current flows in the filament, due to the progression or growth of the normal resistance zone down to the gas-liquid interface, the on-time of the current must be sufficiently long to assure a stable voltage at the lowest possible liquid level. Such a detector system with a fixed time interval is marketed by Intermagnetics General Corporation of Guilderland, N.Y. Even this method of intermittent current flow at regular intervals introduces undersirable heating and evaporative losses.